An artist's impression of Norman Foster's design for a new four-runway airport on the Thames estuary. Photograph. Photograph: Foster And Partners/PA

Downing Street told Boris Johnson on Wednesday that he had killed off any moves towards an airport in the Thames estuary ahead of the next general election after an announcement by the London mayor was seized on by the Liberal Democrats.

David Cameron and George Osborne, who have become alarmed by growing business anxiety about their opposition to expanding Heathrow, had indicated they were warming to the idead of a new airport which would assume Heathrow's role as a hub.

But Johnson was told that he had handed a gift to the Lib Dems, who are opposed to any airport expansion in the south-east, when he briefed the Daily Telegraph that Downing Street had signed up to his proposal.

A Whitehall source said: "Boris is not going to get it. Yes to scoping on a new airport, but he won't get it. The way this has plopped out in the Telegraph, no doubt via Boris's team, has given people a chance to kill it."

The Lib Dems will support a consultation by Justine Greening, the transport secretary, on how to maintain a hub, but they will oppose any new airport.

A Lib Dem source said: "We are against airport expansion in the south-east. We are happy to go along with the consultation, but we have a policy which will not change."

Johnson was upbeat on Wednesday evening, after he outlined his plans to a meeting of the Conservative party's 1922 committee. "We must not count our chickens or over-egg this – to continue with a chicken metaphor – [but] the government is still looking at all the options.

"But George really gets the point about regeneration and jobs in the Thames estuary and the amazing capacity of a big transport hub to drive Britain's competitiveness for a long time to come."

The proposed airport would be an international hub that could open up new routes to south-east Asia and have capacity for connecting flights. At present, the British aviation industry says the nation is losing out to rivals airports such as Schiphol in Amsterdam and Charles de Gaulle in Paris.

The government has ruled out a third runway at Heathrow, as has Labour since leaving power, a current political reality that has been reluctantly accepted by the industry.

Colin Matthews, the chief executive of airports operator BAA, said he was "pleased there was an acknowledgement that there was a need" for more airport capacity, but it would take decades to construct a whole new airport.

"The recognition today is that it matters to the UK economy, to jobs and to growth. There's no reason why any option should be ruled out."

Greening, however, has also ruled out further runways at Gatwick and Stansted in answers to parliamentary questions.

Johnson told the BBC: "You can't go on expecting Britain to compete with France and Germany when we simply can't supply the flights to growth destinations."

Heathrow, he said, was "fundamentally in the wrong place". He said listening to the "Heathrow recidivists" who argue that other options for airport expansion take too long would only lead to paralysis.

In his autumn statement, Osborne indicated the government would consider all options for airport expansion, which he sees as an opportunity for economic regeneration.

The architect Norman Foster has drawn up plans for an airport on the Isle of Grain, the easternmost point of the Hoo peninsula, with four runways and which could handle 150m passengers a year – double the current number passing through Heathrow. It is the latest in a range of proposals dating back decades for the estuary, including a mooted "Boris Island" airport at nearby Shivering Sands.

Aircraft would, however, have to contend with the multitudes of birds that flock in the area – a hazard to aviation and an issue that has brought opposition from environmental groups. The RSPB said it was a vital habitat for wildfowl and wading birds. Balpa, the pilots' union, said it would seek reassurances on the dangers posed by birdstrikes, as well as potential air traffic conflicts with Brussels, Heathrow and London City airports.